The present invention relates to a cathode ray tube, e.g., a color picture tube, and more particularly, to a cathode ray tube capable of effectively decreasing the consumption power of the deflection yoke and the leakage magnetic field generated by the deflection yoke.
FIG. 1A shows a color picture tube as an example of a conventional cathode ray tube. This color picture tube has a vacuum envelope. The vacuum envelope is formed with a substantially rectangular glass panel 1, a glass funnel 2 formed contiguous to the panel 1, and a cylindrical glass neck 3 formed contiguous to the small-diameter end portion of the funnel 2. As shown in FIG. 1B, a substantially rectangular phosphor screen 4 including three dot-like or stripe-like color phosphor layers respectively emitting blue, green, and red light is formed on the inner surface of the panel 1.
An electron gun assembly 7 for emitting three electron beams 6 is arranged in the neck 3. This electron gun assembly 7 is an in-line electron gun assembly that emits the three electron beams 6 arranged in a line on the same horizontal plane.
A deflection yoke 8 is mounted on the outer side of the funnel 2 near the neck 3 side. The deflection yoke 8 generates a pincushion type horizontal deflection field and a barrel type vertical deflection field.
The three electron beams 6 arranged in a line and emitted from the electron gun assembly 7 are deflected by the horizontal and vertical deflection fields generated by the deflection yoke 8 in a horizontal direction H and a vertical direction V. Hence, when they reach the phosphor screen 4 through a shadow mask, the three electron beams 6 arranged in a line converge on the entire portion of the phosphor screen 4, i.e., on the entire screen surface without requiring an extra correction unit, and horizontally and vertically scan the phosphor screen 4, thereby displaying a color image.
A color picture tube having this structure is called a self convergence in-line color picture tube and is widely in use.
In such a cathode ray tube, e.g., a color picture tube, it is important to decrease the consumption power of the deflection yoke 8 which is the maximum power consumption source. More specifically, in order to improve the screen luminance, the anode voltage for finally accelerating the electron beams must be increased. In order to cope with OA equipments, e.g., a HDTV or a High-Definition TV and a PC or a Personal Computer, the deflection frequency must be increased. An increase in anode voltage and an increase in deflection frequency cause an increase in deflection power, i.e., an increase in consumption power of the deflection yoke. In particular, when the electron beams are deflected with a high frequency, the deflection field tends to leak to the outside of the cathode ray tube. For this reason, for a PC in which the operator sits close to the cathode ray tube, regulations against the leakage magnetic field are strict.
In order to decrease the leakage magnetic field, conventionally, a method of adding a compensation coil is generally employed. When, however, a compensation coil is added, the consumption power of the PC increases accordingly. Therefore, in order to decrease the deflection power and the leakage magnetic field, it is preferable to decrease the neck diameter of the cathode ray tube and the outer diameter of the funnel near the neck side on which the deflection yoke is mounted, so that the deflection field efficiently acts on the electron beams.
In the cathode ray tube, when an electron beam is deflected in a direction along the maximum size of the screen, i.e., along the diagonal direction, the deflection angle of the electron beam, i.e., the angle the trace of the deflected electron beam makes with the axis of the neck becomes large. When the deflection angle of the electron beam increases, the electron beam passes closely to the inner surface of the funnel near the neck side on which the deflection yoke is mounted. For this reason, if the neck diameter and the outer diameter of the funnel near the neck side are simply decreased, the outer electron beam 6 bombards the inner wall of the funnel 2 near the neck 3 side, as shown in FIG. 1A. A portion 10 where the electron beam 6 does not reach is thus formed on the phosphor screen 4, as shown in FIG. 1B.
Therefore, in the conventional cathode ray tube, the neck diameter and the outer diameter of the funnel near the neck side cannot be simply decreased. Accordingly, it is difficult to decrease the deflection power and the leakage magnetic field. If the electron beams 6 continue to bombard the inner wall of the funnel 2 near the neck 3 side, the temperature of this portion rises to melt the glass. Then, a portion of the inner wall of the funnel becomes thin, and the funnel may break at this portion.
In order to solve these problems, Jpn. Pat. Appln. KOKOKU Publication No. 48-34349 discloses a cathode ray tube 12 as shown in FIG. 2A. This tube is developed based on the fact that when drawing a rectangular raster on a phosphor screen, a passing region which is defined by the trace of an electron beam passing inside the funnel near the neck side on which the deflection yoke is mounted also becomes substantially rectangular. More specifically, in this cathode ray tube 12, as shown in FIGS. 2B to 2F showing the sections of the cathode ray tube 12 taken along the lines IIB--IIB to IIF--IIF, respectively, the section of a funnel 2 near the neck 3 side toward the panel 1 side, on which the deflection yoke is mounted, gradually changes from a circular shape to a substantially rectangular shape through an elliptic shape.
In a cathode ray tube whose funnel near the neck side on which a deflection yoke is mounted is formed with sections as shown in FIGS. 2B to 2F, the inner diameter of the corner portion, i.e., a portion near the diagonal axis (D axis), where the electron beams tend to land, becomes large, as shown in FIG. 3, as compared to that in a cathode ray tube whose funnel 2 near the neck side remains circular. This prevents the electron beams from impinging on the inner wall of the funnel.
In the cathode ray tube having a structure as shown in FIGS. 2B to 2F, its inner diameter near the major axis, i.e., the horizontal axis (H axis), and its inner diameter near the minor axis, i.e., the vertical axis (V axis), become shorter than in the cathode ray tube whose funnel 2 near the neck side remains circular. This aims at setting the horizontal deflection coil and the vertical deflection coil of the deflection yoke to be closer to the passing region of the electron beams in order to efficiently deflect the electron beams, thereby decreasing the deflection power.
In this cathode ray tube, however, when the section of the funnel near the neck side on which the deflection yoke is mounted becomes closer to a rectangle, the atmospheric pressure resistance decreases, and safety is impaired. Therefore, in practice, the shape of the funnel near the neck side must be appropriately rounded, and the deflection power and the leakage magnetic field cannot thus be decreased sufficiently.
As described above, it is very difficult to realize a decrease in deflection power and leakage magnetic field of a cathode ray tube while satisfying demands for a higher luminance and a higher frequency required by a display equipment, e.g., a HDTV and a PC. Conventionally, in a structure proposed to reduce the deflection power of a cathode ray tube, the shape of a funnel near the neck side toward the panel side, on which a deflection yoke is mounted, gradually changes from a circular shape to a substantially rectangular shape through an elliptic shape.
When, however, the section of the funnel near the neck side becomes closer to a rectangle in this manner, the atmospheric pressure resistance suffers, impairing the safety. Therefore, in practice, the shape of the funnel near the neck side must be appropriately rounded, and the deflection power cannot thus be decreased sufficiently. Also, at the time the above mentioned reference was published, the simulation techniques for designing the shape of the envelope of the cathode ray tube were not mature yet, and electron beam trace analysis and deflection field analysis as accurate as those nowadays done could not be performed. Therefore, a funnel that could decrease the deflection power and the leakage magnetic field while maintaining the atmospheric pressure resistance could not be designed.